I have an administrative contact for my request, how to deal with that?

You can ask for an institutional invoice directly in the process of posting a request by checking the box "This request will be paid by my Research support office.
Please provide administrative contact information needed to set up the service account for your institution (see FAQ)" (please see the screenshot below). If you check this box, please do not forget to provide the full name of your institution, its address and the VAT number (if applicable).